1. Field of the Invention
The present invention relates to a carburizing method and a carburizing apparatus mainly for a member made of steel and more particularly to an economical carburizing method and carburizing apparatus both capable of carrying out carburizing treatment with high reproducibility and giving high carburizing quality.
2. Description of the Related Art
As conventional carburizing methods, there exist a gas carburizing method, a plasma carburizing method, a vacuum carburizing method and the like. Among them, the gas carburizing method is a method for carrying out carburization while controlling the atmosphere, so that the carbon concentration in the surface of an object to be treated can stably be controlled. For that, since the carburizng treatment is excellent in the reproducibility and the quality, the method is most widely applied to industrial machinery parts of such as automobiles.
However, the gas carburizing method has problematic disadvantages: that the use amount of a carburizing gas is high; there is danger at the time of burning an exhausted gas; intergranular oxidation takes place in the surface of an object to be treated; carburizing at a high temperature is difficult; and the like.
Although the plasma carburizing method is advantageously capable of carburizing even materials hard to be carburized such as stainless steel, Ti alloys, and the like, it has such problems that an apparatus is costly; carburizing treatment cannot be carried out while objects to be treated being arranged densely; and the quality of objects subjected to the carburizing treatment is unstable, resulting in inferior reproducibility of the carburizing treatment because of the absence of atmosphere control.
The vacuum carburizing method can broadly be divided into two systems. One system, which has been employed for long, is to carry out carbonization using a hydrocarbon such as CH4, C3H8, C4H10 as a carburizing gas under a pressure as high as about 10 to 70 kPa. The vacuum carburizing method of the old system has such advantages as no intergranular oxidation taking place, capability of carrying out carburizing treatment at a high temperature and possibility to be carried out in a short carburizing time. However, sooting is so intense that a troublesome maintenance work is frequently required and the working environment for the maintenance is inferior. Further, since atmosphere control is not carried out, there is another problem that reproducibility of the carburizing treatment is low.
On the other hand, a vacuum carburizing method of a new system is a system to carry out carburizing treatment using a hydrocarbon such as C3H8, C2H2, C2H4 as a carburizing gas under a pressure as high as about 10 kPa or lower. The vacuum carburizing method of this system has advantages that the sooting is slight as compared with the above described old system; no intergranular oxidation is caused; and high temperature carburization is possible to make the carburizing time short.